1. Field of the Invention
The present invention is directed to a method for generating excited neutral particles for etching and deposition processes in semiconductor technology on the basis of a plasma discharge fed by microwave energy, whereby microwave energy having a specific frequency is generated, is coupled into a waveguide system and, as a standing transversal electrical wave, is concentrated therein at predetermined locations, and whereby process gases intended for excitation are conducted through the waveguide system with a plasma discharge tube aligned in the direction of the electrical field of the wave, whereby a plasma is ignited and excited particles are generated. The invention is also directed to an apparatus for the implementation of this method.
2. Description of the Related Art
A method for generated excited neutral particles of said species is known from T. Sugano, "Applications of Plasma Processes to VLSI Technology", Wiley-Interscience, New York, 1985, Sections 2.2 and 2.3 (particularly see 2.2.2).
In addition to lithography and doping techniques, etching and deposition techniques are fundamental processes that are employed again and again in the process sequence for manufacturing LSI circuits of silicon substrates (see, in general, "Technologie hochintegrierter Schaltungen", D. Widmann, H. Mader, H. Friedrich, Springerverlag 1988, particularly Sections 3.1.1 and 5.2..2-4). For example, an important method is vapor phase deposition, also called CVD, whereby it is presently often standard to undertake the excitation of the initial reaction gases to form dissociated, reactable constituents and the initiation of the deposition reaction primarily not on the basis of an elevation of the temperature (of the silicon wafer) to approximately 1000.degree. C. but to undertake this by a plasma or by a high-energy radiation. Dry etching processes, for which the formation of a gaseous, volatile reaction product is a prerequisite, also usually only proceed spontaneously, i.e. without 10 the application of external energy when the gases are already present in atomic form.
It is obviously a critical concern for the successful implementation of such etching and deposition processes to generate high-energy and, therefore, reactable neutral particles, particularly radicals, with an adequately high efficiency. The technical solution of this demand is increasingly being striven for simultaneously with satisfaction of the farther-reaching demands for a prevention of the influence of electrical fields and charged particles on the substrate to be processed and for an optimally broad range of working pressure for the etching and deposition processes.
In order to protect the substrates against undesired electrostatic fields and against ions, which are always co-produced in addition to the neutral particles in the standard dissociation of process gases in a plasma gas discharge, it is known to spatially separate the generation of excited neutral particles from their employment in an etching or deposition process occurring in a reaction chamber (downstream process). Due to the short life span of the charged particles, their concentration decreases greatly immediately after the excitation zone in the downstream method, whereas the excited neutral particles--as a consequence of their substantially longer life span-reach the reaction chamber in what is a suitable concentration for many applications via a suitable feeder. Magnetron generators having a working frequency of a number of GHz are often utilized as energy sources for the radio-frequency plasma discharge in order to generate corresponding microwaves. This energy is coupled into a cavity resonator or, respectively, into a waveguide system and--on the basis of suitable dimensioning and tuning--is concentrated therein at specific locations in the form of a standing wave. A plasma discharge tube is then usually conducted through the waveguide system at one of these locations, i.e. exactly at a location at which the energy of the standing wave concentrates. In this way, radicals having a long life span can be generated in process gases that are supplied to the plasma discharge tube and can be subsequently conveyed to the reaction chamber with a feeder. The localization of the energy of the standing wave at the correct location is basically unproblematical; however, a substantial part of the energy is not converted to excitation but is reflected untuned and must be absorbed in the waveguide, usually in a water load, so that the magnetron is not damaged (see Sugano, Opp. Cit., Section 2.2.2).
The described, only partial conversion of the available microwave energy proves problematical particularly in the light of the afore-mentioned demand for a broad range of working pressures insofar as it is precisely the low pressure range below approximately 13, particularly below 1.3 Pa, that is interesting and advantageous for semiconductor technology.
Low pressures are of significance, for example, for surface-controlled CVD processes in order to avoid depositions having undesired layer properties. In etching processes, too, a high etching rate and the prevention of microload effects, i.e. a local etching rate dependent on the environment, can often only be realized at extremely low pressures. However, ignition difficulties in the plasma discharge already begin to arise in the pressure range below 13 Pa, since the excitation density and, thus, the efficiency of the generating decrease too greatly.
It is in fact known (see Sugano, Op. Cit., Section 2.3.2) to also stabilize the plasma in the pressure range below 13.times.10.sup.-2 Pa with the assistance of being enclosed in a magnetic field whose cyclotron frequency is in resonance with the frequency of the microwaves (ECR method). As proceeds, for example, from the article "Downstream Plasma Etching and Stripping", by J. M. Cook, Solid State Technology, Apr. 1987, particularly 150, excited neutral particles cannot be made available in adequate numbers and with adequate density overall with such methods, i.e. particularly at the wafer itself. This is not surprising in view of the fact that only at most 30% of the microwave energy in the discharge is converted even given improved ECI methods.
Taking a method that can be manipulated into consideration, moreover, it is also not possible to substantially increase the infed microwave energy itself, usually approximately 1 kW, in order to enhance the efficiency of the generating.